Astronomers may have solved a
cosmic chicken-and-egg problem
-- the question of which formed
first in the early Universe --
galaxies or the supermassive
black holes seen at their cores.
"It looks like the black holes
came first. The evidence is piling
up," said Chris Carilli, of the
National Radio Astronomy
Observatory (NRAO). Carilli
outlined the conclusions from
recent research done by an
international team studying
conditions in the first billion
years of the Universe's history
in a lecture presented to the
American Astronomical Society's
meeting in Long Beach,
California.
Earlier studies of galaxies and
their central black holes in the
nearby Universe revealed an
intriguing linkage between the
masses of the black holes and of
the central "bulges" of stars and
gas in the galaxies. The ratio of
the black hole and the bulge
mass is nearly the same for a
wide range of galactic sizes and
ages. For central black holes
from a few million to many
billions of times the mass of our
Sun, the black hole's mass is
about one one-thousandth of
the mass of the surrounding
galactic bulge.
"This constant ratio indicates
that the black hole and the bulge
affect each others' growth in
some sort of interactive
relationship," said Dominik
Riechers, of Caltech. "The big
question has been whether one
grows before the other or if
they grow together,
maintaining their mass ratio
throughout the entire process."
In the past few years, scientists
have used the National Science
Foundation's Very Large Array
radio telescope and the Plateau
de Bure Interferometer in France
to peer far back in the 13.7
billion-year history of the
Universe, to the dawn of the
first galaxies.
"We finally have been able to
measure black-hole and bulge
masses in several galaxies seen
as they were in the first billion
years after the Big Bang, and
the evidence suggests that the
constant ratio seen nearby may
not hold in the early Universe.
The black holes in these young
galaxies are much more massive
compared to the bulges than
those seen in the nearby
Universe," said Fabian Walter of
the Max-Planck Institute for
Astronomy (MPIfA) in Germany.
"The implication is that the black
holes started growing first."
The next challenge is to figure
out how the black hole and the
bulge affect each others'
growth. "We don't know what
mechanism is at work here, and
why, at some point in the
process, the 'standard' ratio
between the masses is
established," Riechers said.
New telescopes now under
construction will be key tools
for unraveling this mystery,
Carilli explained. "The Expanded
Very Large Array (EVLA) and the
Atacama Large Millimeter/
submillimeter Array (ALMA) will
give us dramatic improvements
in sensitivity and the resolving
power to image the gas in these
galaxies on the small scales
required to make detailed
studies of their dynamics," he
said.
"To understand how the
Universe got to be the way it is
today, we must understand
how the first stars and galaxies
were formed when the Universe
was young. With the new
observatories we'll have in the
next few years, we'll have the
opportunity to learn important
details from the era when the
Universe was only a toddler
compared to today's adult,"
Carilli said.
Carilli, Riechers and Walter
worked with Frank Bertoldi of
Bonn University; Karl Menten of
MPIfR; and Pierre Cox and
Roberto Neri of the Insitute for
Millimeter Radio Astronomy
(IRAM) in France.
The National Radio Astronomy
Observatory is a facility of the
National Science Foundation,
operated under cooperative
agreement by Associated
Universities, Inc.